1. Field of the Invention
This invention relates to a mine roof support truss and, more particularly, to a truss system that is economically fabricated and efficiently installed to produce a desired tension for supporting a wide variety of mine roof conditions.
2. Description of the Prior Art
Truss-type mine roof supports are well known in the art of supporting the roof of an underground passageway, such as a mine passage. A basic truss system includes one or more rods extending horizontally the width of the mine passage adjacent the roof and connected at their ends to anchor bolts which extend at an angle adjacent the ribs of the passageway into the rock strata over a solid pillar. The rods are tensioned and vertical components of compressive forces are transmitted into the solid material over the pillars as opposed to the unsupported rock material immediately above the passageway.
With this arrangement, a truss system shifts the weight of the rock strata from over the mined out passageway back onto the pillars. The desirability of truss systems has been enhanced by the development of roof bolting machines that can convert from vertical to angle drilling. Conventionally, holes are drilled into the mine roof at a 45.degree. angle from the horizontal adjacent to the mine rib so that the holes extend into the supported rock structure of a pillar. To insure adequate anchorage over the pillar at the ribline, the bolts extend up to six or seven feet into the supported structure of the pillar.
Once the angle holes are drilled into the strata over the pillars at the ribline, anchor bolts are inserted in the drilled holes and are secured in place using mechanical expansion shell assemblies in combination with resin. This arrangement insures adequate anchorage over the ribline for bolts that extend in length up to six or seven feet. Before the bolts are inserted in the drilled holes, truss shoes or bearing blocks are positioned on the bolt at the emergent end of the bolt from the hole. As the bolts are securely anchored in the bore holes, the bearing surfaces of the truss shoes or bearing blocks are compressed into engagement with the mine roof.
For an uneven mine roof or a roof having severely potted areas, the truss shoe or bearing block must have a sufficient bearing surface to contact the mine roof so that the plate does not slip and the truss shoe is correctly positioned for engagement with the horizontal truss members. Once the truss shoes or bearing blocks are securely positioned at the mine roof adjacent the ribs, the horizontal truss members are assembled and connected to the truss shoes. The truss members are tightened to a preselected torque to exert tension on the truss members so that the weight of the rock strata over the mined out area beneath the roof is shifted along the horizontal truss members upwardly into the solid rock strata over the pillars at the ribline.
A wide variety of truss hardware is commercially available to form a truss system between the anchored angle bolts. The truss hardware is connected under tension to the truss shoes or bearing blocks that are held tightly against the mine roof by the anchored angle bolts.
U.S. Pat. Nos. 4,601,616 and 4,630,974 are examples of mine roof truss systems that utilize L-shaped bearing plates for connecting the horizontal truss members to the anchored angle bolts. The bearing plates include a horizontal member that bears against the mine roof and receives the anchor bolt. A vertically depending portion of the plate has a transversely extending opening through which a tie rod or cord of the truss extends. An enlarged bolt head abuts the vertically depending portion, and the ends of the tie rod extending from the bearing plates are coupled together. A preselected torque is applied to the coupled ends of the tie rods to create the desired degree of tension in the truss system.
The above described truss system connects the coupled truss members to the bearing plates and requires that the truss members be extended through holes in the bearing plates and secured thereto by nuts threaded onto the ends of the truss members. The bearing plates are positioned closely adjacent the ribline which may provide insufficient space to extend the elongated rods through the holes. Consequently, the truss members may be required to be connected to the bearing plates before the plates are anchored to the passage roof. This can be a time consuming and difficult task.
An alternate approach to connecting truss hardware to anchored truss shoes or bearing plates is disclosed in U.S. Pat. No. 4,934,873 in which the roof anchor has an eye-bolt forged on the end of the roof anchor. A U-bolt passes through the eye of the bolt head and is then bolted to a retainer or block member that extends transversely between the legs of the U-bolt. An elongated tension bolt having an enlarged head at one end and a threaded end at the opposite passes through the header of one U-bolt so that the enlarged head abuts the header. The opposite threaded end of the tension bolt passes through the U-bolt header at the opposite rib and receives a nut which is threaded onto the tension bolt. By tightening the tension bolt through the nut on the threaded end, the eye-bolt ends of the anchor bolts are drawn toward one another to place the truss members in tension and support the mine roof. The truss members are securely connected to one another as well as to the anchored eye-bolt ends.
Truss systems have also been proposed to facilitate ease of assembly and disassembly of the truss hardware to the anchored truss shoes without requiring that the truss hardware be connected to the truss shoes before they are positioned at the mine roof. U.S. Pat. Nos. 4,596,496 and 5,026,217 disclose truss shoes having a J-shaped lip that receives the closed or arcuate end of a U-bolt. The J-shaped lip of the truss shoe includes a recess for receiving the end of the U-bolt. This facilitates ease of assembly and disassembly of the U-bolt on the truss shoe without requiring the U-bolt to be threadedly connected to the shoe or extended through an opening in the shoe. The end of the U-bolt is looped onto the recess and is permitted to hang freely downwardly on the shoe and then pivoted upwardly into a horizontal position for connection of the truss members. However, the U-bolt is not securely connected to the truss shoe until it is placed in tension. When the connected truss members are tensioned, the U-bolts are drawn together and securely retained on the truss shoes. When the U-bolts are released from connection, the bolts can be pivoted downwardly to hang from the truss shoes. However, if the truss shoes are inclined from the horizontal due to an uneven roof or the tension on the bolts is reduced or the bolts shift horizontally beyond the bearing surface of the J-shaped lip, the U-bolts can fall from the shoes. Therefore, measures must be taken to hold the U-bolts on the shoes before they are swung into a horizontal position. Then, once in the horizontal position, the required tension must be maintained on the bolts to prevent their horizontal shifting.
While it has been proposed to securely connect truss members to truss shoes so that the truss members will not be displaced and can be vertically retained preliminarily to connection and tensioning, the mechanical connections required to secure the truss members to the shoes necessitate additional hardware and time of installation. On the other hand, for truss shoes having a J-shaped lip, while the U-bolts are easily looped into position on the truss shoes, the U-bolts can become displaced from the truss shoes unless sufficient tension is maintained on the U-bolts to retain them secured to the truss shoes.
Therefore, there is need for a truss system that permits ease of installation of U-bolts to truss shoes and also provides means for efficiently retaining the U-bolts on the truss shoes in both a vertically hanging position and a horizontal position as well. In order to facilitate connection of the truss shoes to the truss members and permit adjustment in the length of the connected truss members for mounting on the truss shoes, the U-bolts must be retained for horizontal movement on the truss shoes. This arrangement would serve to decrease the installation time of the truss. It would then be possible to install a truss system on-cycle with the mining operation, particularly when the roof control plan specifies that trusses be installed on centers as close as one foot.